A sample of the luminescent solar concentrator, illuminated under ultraviolet light. [Image: Uwe Kortshagen, University of Minnesota]
Italian and U.S. scientists have developed a method for creating large-area, flexible transparent sheets—built out of polymers infused with silicon quantum dots (QDs)—that can effectively capture solar energy and channel it to photovoltaic cells at the sheet’s edges (Nat. Photon., doi: 10.1038/nphoton.2017.5). The team believes that the technique could offer a practical route toward inexpensive photovoltaic windows, which could materially extend the benefits of solar power in dense metro areas.
Solar in the city
While photovoltaic cells, in silicon and other materials, continue to decline in cost and make inroads in a wide variety of industrial, commercial and residential settings, dense urban environments have posed a bit of a stumbling block for the technology. The rooftops of a typical skyscraper lack sufficient surface area for solar cells to make a significant dent in the building’s energy needs. And other efforts to build solar into urban buildings can compromise their aesthetics and design flexibility.
One potential way around this is to design building windows that can act as luminescent solar concentrators (LSCs). In this scheme, semi-transparent window material, doped with fluorophores or emissive materials, captures a share of direct or diffused sunlight and re-emits the light. The thin window, by virtue of its refractive-index contrast with the surrounding air, acts as a waveguide that channels the re-emitted light, sending it to the edges of the window, where it is harvested by small photovoltaic cells discreetly hidden in the window frame.
The approach has several notable advantages. The large window area, for example, offers the possibility of capturing and rechanneling a substantial harvest of solar photons. And, given suitably flexible materials, the approach could give architects and engineers considerable design freedom in creating so-called zero-energy buildings.
Finding the right emitter
What has held the technology back, however, has been a lack of suitable emitters. Many conventional fluorophores have limited spectral coverage—and, even worse, their absorption and emittance spectra tend to overlap. That means that a large share of the re-emitted solar photons can be reabsorbed by the embedded fluorophores, and might never make it to the photovoltaic cells at the window’s edges, a deal-killer for a photovoltaic window’s power-conversion efficiency.
A research team from the Università degli Studi di Milano-Bicocca, Italy, and the University of Minnesota, USA, looked for a better emitter in an up-and-coming material: silicon QDs. These indirect-bandgap semiconductor nanoparticles can be tuned to harvest energy over a wide part of the visible spectrum, into the near infrared, and to emit at a narrow wavelength band, dependent on the size of the particle.
More important, the separation between the absorption and emission spectra can be tweaked, so that the dots re-emit radiation at a wavelength safely away from their absorption peaks. That means that most of the re-emitted photons can be expected to make their way through the waveguide and be harvested by the solar cells at the edges, rather than being lost to reabsorption. An additional advantage of silicon QDs is that they are made from one of Earth’s most abundant materials, which offers a clear cost advantage relative to other QDs candidates made of rare or potentially toxic substances.
Efficiency and flexibility
The Italian-U.S. team put these ideas into practice by creating a slab of polylauryl methacrylate (PLMA), a transparent polymer, containing dispersed silicon QDs. The dots were engineered to absorb strongly at wavelengths below 600 nm, with negligible absorption at longer wavelengths, and to re-emit in a much narrower window at around 830 nm. They then illuminated the slab using a solar simulator with a flux of 100 mW/cm2, and measured the effect on solar cells installed at the slab’s perimeter.
The researchers found that the setup converted 2.85 percent of the incident energy into electrical energy at the cells, despite the fact that the material was highly transparent across the visible spectrum—a clear indication that the QDs and the thin-slab waveguide were doing their work. Analysis of the photoluminescence spectra at the slab edges suggested virtually no loss of re-emitted photons to reabsorption. And the team established that the system worked as well in flexible, curved window surfaces as in flat ones, a nice plus for architectural design flexibility.
Additional simulations, say the scientists, point to the possibility of a power-conversion efficiency of 5 percent or more for windows as large as a meter square, given further tweaks in particular to the QDs to improve their emissivity. Those levels—while nowhere near the efficiencies of dedicated solar panels—could be enough open a path toward turning the windows of urban buildings into meaningful electricity generators.